This invention relates to microelectronic device and circuit fabrication, and in particular to a method of bonding microelectronic chips to various surfaces.
In many cases of microelectronic device and circuit fabrication, it is necessary to bond chips to surfaces such as headers, lead frames, ceramic substrates, and mounting studs where only extremely small areas are available for bonding. Basically, three approaches are followed when a soft, ductile bonding material such as a solder is used to form the bond. In one approach, the bonding material is in the form of a wafer preform which is placed on the bonding surface (see, for example, U.S. Pat. No. 3,781,978, issued to Intrator, et al.). Although adequate for many purposes, this method cannot be used effectively when soft ductile material, such as indium, is desired in thicknesses of 4 microns or less since physical handling of the preform is extremely difficult. An alternative method is to evaporate the bonding material into the bonding surface. This technique is also effective for most applications. In certain areas, however, such as bonding laser chips to rate the bonding material into the bonding surface. This technique is also effective for most applications. In certain areas, however, such as bonding laser chips to mounting studs, the evaporated metal tends to spatter upward from the sides of the stud during the bonding operation resulting in contamination of the mirror surfaces. A third approach involves the use of epoxy compounds deposited as a solid onto the bonding surface. Presently available epoxies cannot be used conveniently in certain applications, such as laser bonding, because the thermal impedance and the curing temperatures tend to be too high.
The invention is therefore directed to the problem of bonding microelectronic chips to bonding surfaces by means of a thin, soft ductile bonding layer where a preform is too small to be handled economically and the use of an evaporation procedure could result in device contamination.